Large turbine generators are typically cooled with a light density gas. Hydrogen (H2) has been widely used as a coolant due to its desirable thermophysical properties including low windage friction, high heat dissipation capability and high resistance to corona discharge when compared to other cooling gas options. Additionally, H2 has the advantage of being readily accessible and inexpensive.
Leakage of H2 may prevent the turbine generator from operating efficiently, and in some cases may create power generation outages. Among possible areas of H2 leakage around a turbine generator, are leaky spots at the wave stator casing including high voltage bushings and joints. Leaks may also occur around the interfaces of the cooler, welds, bolt heads and endshield. The bearing enclosure in the outer end shields, the rotor terminal packing, as well as drill holes made for instrumentation plug-ins may also be susceptible to leaks. Other air-tight transitions and welding joints may be sources of leaks, as well as the seal oil drain system, gas piping, and hydrogen cabinet. If the generator is a water cooled generator the stator liquid cooled windings also may be a source of leaks.
H2 leaks are difficult to detect because H2 is colorless and odorless, and because of its low density it dissipates quickly when it leaks into the atmosphere. The technical challenges in monitoring and detecting a potential H2 leak lie in identifying the exact location of H2 leaking in a turbine generator, especially in inaccessible and space limited areas.
Conventional turbine generator leak detection methods require the purging of the turbine generator with air and thereafter bringing it up to normal operating pressure. Then a long check list of areas to be examined and algorithm of step-by-step elimination are used. Each cycle of the testing requires monitoring for at least 24 hours. Standard formulae for volume, temperature and pressure are used to calculate loss of air over each period, and then a conversion is made to determine the equivalent H2 loss. If the leakage is higher than recommended a variety of methods of leak detection have been used.
For example, a bubble test may be performed using soapy water or a similar detergent solution applied over all the accessible areas of possible leaks. If the leakage is inward in the stator liquid cooled windings, a flammable gas detector may even be used at the vent. The leak rate is determined by a “bag” test method. The process is time consuming because each time a leak is located in those accessible areas and repaired, another air test is required to confirm that the H2 system is at an acceptable leakage rate. Each test cycle adds 24 hours to the outage.
Another approach is to use a halogen leak detector designed for detecting leaks in a pressurized system where halogen compound gases (such as Freon 12) are used as a tracer gas to check for leaks. The exterior of the system is then scanned with a sniffer probe sensitive to traces of the halogen-bearing gas. The principle is based on the increased positive ions (K or Na) emission because of sudden halide composition presence.
Yet another approach is to use a flammable gas detector designed to display a reading based on a percentage of the lower explosive limit of a hydrogen-air mixture (4% hydrogen in 100% air—therefore a 100% scale reading indicates a 4% or greater concentration of hydrogen in air).
Yet another approach is to use an ultrasonic leak detector that utilizes the ultrasonic energy generated by molecular collisions as gas escapes from or enters a small orifice. Pressurized gas proceeds from the leak locale and are detected with a sensitive microphone (typically about 40 000 Hz).
Multiple gas detectors have also been used. This type of leak detector is sensitive to a wide range of different gases in air. It detects inert gases (such as helium), flammable gases (hydrogen), corrosive gases (ammonia, chlorine), halogens (Freon) and also carbon dioxide.
Another approach has been to add odorants indicate the general area of the leak, after which the leak may be traced to its source by one of the foregoing methods.
All conventional methods of leak detection require the detector to be in close proximity to the source of the leak and take considerable time to implement. Most of the conventional methods use close or near contact “sniffer” technology and probes. These methods are painstakingly time consuming and in some cases miss the gas leaks. If the inaccessible H2 sealing system or constrained space is the source of a possible leak, considerable effort to disassemble the turbine generator may be needed, commonly resulting in delaying the schedule several more days. Values approaching $1 MM loss of operating revenue per day have been reported by power producers when a turbine generator is off-line.
Long wave gas detection cameras (detector response of 10-11 μm) have been used in the electrical distribution industry to detect leakage of Sulfur Hexafluoride (SF6) from high voltage switchgear and transformers. It has also been proposed to use SF6 as a tracer gas in finding H2 leaks in power plant generators in combination with backscatter/absorption technology. The backscatter/absorption leak detection process uses an active scanning laser to provide a directed energy source to irradiate a target area. The laser beam is reflected back to the source camera tuned to a specific frequency band. SF6 has high affinity to absorb this frequency of energy and appear as a dark cloud on the camera monitor. The camera monitor provides a direct indication of how serious the leaks are by the size and darkness of the tracer gas cloud.
The major issues associated with the use of SF6 as a tracer gas relate to environmental, health, and safety concerns and the potential deterioration of turbine generator insulation systems and retaining rings. SF6 is a potent greenhouse gas with a ‘global warming potential’ (GWP) of 23,900 and an atmospheric lifetime of 3,200 years. The release of 1 kg of SF6 into the atmosphere has the same impact as a release of 23,900 kg of CO2. Release of SF6 to the environment after detection, or the remaining residue at ppm (parts per million) level is of environmental, health, and safety concern. Additionally, in the presence of potential corona activities and thermal stress during turbine generator operations, SF6 can decompose into harmful byproducts. These byproducts include HF, SF4, SO2, and SO2F2 which are toxic gases. In the presence of moisture, the primary and secondary decomposition products of SF6 form corrosive electrolytes which may cause damage and operational failure to an H2 cooled turbine generator. For example, SF6 and its degradation byproduct have known corrosion effects on generator field retaining ring material whose main composition is 18Cr-18C stainless steel.
Existing methods do not provide a remote, sensitive, accurate, safe, fast and non-corrosive detection capability adaptable to being integrated with an on-line control system.